Electronic, and particularly digital, processing circuits have been developed to store, retrieve and enhance the display of optical images, especially where the object being captured is difficult to image, or the image and transmission medium are characterized by distortions or noise. As one particular example, a new type of thermal imaging system, called a "Thermodynamics Infrared Imaging Sensor", has been developed to capture images of the heat pattern of an object, such as for night vision or target identification. Such thermal imaging systems are described, for example, in U.S. Pat. No. 4,788,428 to Metcalf et al., which is incorporated herein by reference.
As illustrated in FIG. 1, a typical thermal imaging system employs a laser beam from a laser source 10 which is passed through a large-area interferometer 11 which includes an imaging mirror 16 that is subtly deformed by infrared energy from the object imaged on the mirror 16 through an infrared (IR) objective lens 18. The heat patterns of the object are thus used to modulate the transmitted laser beam. Laser light reflected from the mirror 16 and the interference beam are combined to generate an interference pattern at a camera image plane 12 of the interferometer 11. The interference pattern at the camera image plane 12 corresponding to the thermal image is read by a camera or detector 12. The input image signals from the camera 12 are processed by an image processing circuit 13 to generate video signals to be displayed on a CRT 14 and/or stored in a storage unit 15.
A difficult technical problem for this and other, similar imaging systems is that the object may provide only a very faint image on a large, non-uniform fixed background pattern (FBP). Signal processing is required to subtract the FBP image in order to provide a readable differential image. However, the desired operational requirements for obtaining a differential image can impose a severe task on conventional signal processing systems because of low sensitivity and/or high noise. For example, the camera in the interferometer sensor for military applications is required to detect an image approximately 1000 times fainter than the FBP, for a sensitivity of one degree Centigrade difference. For a temperature difference sensitivity of 0.1 degree Centigrade, the image is approximately 10,000 times fainter than the FBP.
As shown in FIG. 3, another application of the camera 12 is as a laser targeting system. Such a system does not use a laser interferometer 11, and is not part of a passive infrared imaging system as shown in FIG. 1. Instead, the camera 12 is used to look directly for scattered light reflected from the object by a targeting laser 10. The light image from the object is focused by the objective lens 18' and filtered by the laser band filter 19 and passed to the camera image plane. The laser targeting system requires a very sensitive differential camera if the targeting laser light scattered from the object is much weaker than the general scene illumination (after narrow band filtering), e.g., particularly with a low-power targeting laser on a bright, sunny day. The filtered object input without the scattered laser light is referred to as the fixed background pattern (FBP). The FBP must be subtracted by the differential camera, just as in the interferometer imaging application. With the interferometer, however, the FBP is inherent within the imaging device, while in the targeting application, it is part of the input image signal of the object.
Conventional differential electronic imaging systems operate by storing an image of the FBP in electronic memory and subtracting it during system operation to yield the differential image. The FBP must be stored as a separate value for each pixel, since the expected non-uniformity in the FBP can be much greater than the image to be enhanced. Furthermore, the detection, storage, readout, and subtraction must be done with high accuracy and low noise due to the weakness of the signal compared to the FBP. A minimum of 12 bits of accuracy in FBP storage, and a maximum overall signal/noise difference of 4000 times are required for one degree Centigrade sensitivity in the interferometer application.
Examples of conventional schemes for differential image processing are illustrated in FIGS. 2A and 2B. The systems in FIGS. 2A and 2B require the FBP to be stored in digital form and retrieved for generating the differential image as a video output, using serial A/D and D/A signal conversions. Both systems use a PIN diode array camera 20, instead of a CCD camera, because of the lower readout noise level. The image plane captured by the PIN diode array 21 is scanned by an array scanning and video conversion unit 22. In the system of FIG. 2A, the object image and the FBP image are converted to digital form by the fast 12-bit A/D converter 23. The FBP digital data are stored in a RAM 24, and then retrieved for subtraction from the object image at the summing node 25 during system operation. The digital difference data are scaled and then converted to analog video signals by the 8-bit- D/A converter 26 for video output. In the system of FIG. 2B, only the FBP image is converted to digital form by the slow 12-bit A/D converter 23'. The stored FBP digital data are converted back to analog form by the fast 12-bit D/A converter 27, and subtracted from the analog object image signal at the summing node 28 for video output. The asterisks in the figures indicate the FBP storage path, which must be reset with an updated FBP image periodically, such as by switching off the object image with a chopper 17 as shown in FIG. 1 or FIG. 3.
The performance of systems using the PIN diode array camera, as shown in FIGS. 2A and 2B, is uncertain due to possibly unacceptable readout noise levels. Also, the analog subtraction scheme shown in FIG. 2B is problematic because the negative FBP video signal is regenerated from digital data, i.e. converted twice, and may not have the same shape between samples as the original image.
Furthermore, all of these schemes are subject to FBP drift after the FBP has been stored in memory. This can happen, for example, in the thermal imager if the system temperature changes unevenly. To prevent this, the system would have to be carefully constructed so that temperature changes would occur slowly enough to allow approximate equilibration across the image plane. Frequent recalibration of the FBP would be required using the chopper to switch from the object signal to the background.
Another serious problem is the data processing bottleneck of the 12-bit A/D and D/A converters. Each pixel requires one data conversion per video scan period. As a result, there is a direct tradeoff between the number of pixels, frame rate, and background subtraction accuracy (system sensitivity). For example, a 256.times.256 pixel system with one degree Centigrade sensitivity (requiring 12-bit accuracy), and a frame rate of 20/second, would require a data conversion time of 0.7 usec. This is at the upper speed limit for existing 12-bit D/A converters, thus making it difficult to implement and difficult to improve performance. Existing 12-bit A/D converters do not have this speed, so the scheme of FIG. 2A cannot be implemented at all at this performance level.
It is therefore a principal object of the invention to provide a differential electronic imaging system which has low noise, eliminates the data processing bottleneck, and permits high data conversion accuracy for better sensitivity and image resolution. It is a further object to provide a system which avoids the need for frequent FBP recalibration. A specific object also is to provide a highly sensitive differential imaging system which may be used to locate extremely weak signals in a strong FBP.